1. Field of the Invention
This application relates to the field of coloring plastics and particularly to liquid colorant compositions for use in coloring plastics.
2. Description of the Prior Art
It has been common industry practice to color thermoplastic resins during the processing steps, such as extrusion, injection or blow molding, and calendering, by using either color concentrates or so-called "dry color".
Color concentrates are dispersions or color pigment in a resin, usually the same resin as the clear resin the processor is using and usually in pellet form, at pigment concentrations of from about 10-60 %. The processor must either preblend the color concentrate pellets with the clear resin pellets or meter the color concentrate pellets into the clear resin pellet feedstream at from 0.5 to 10% by weight, in order to reduce the pigment concentration in the finished processed article to the desired level, usually from about 0.2 to 3.0%. Use of color concentrates has a number of disadvantages, two major ones being set forth in the paragraphs below.
If preblending is used this adds an additional processing step, with attendant cost, since the color concentrate pellets and clear resin pellets must be tumble-blended and delivered batch by batch to the processing equipment.
Metering of resin color concentrate pellets directly into the clear resin pellet feedstream at the bottom of feed hoppers of, for instance, extruders or injection molders, has also been practiced for some time. Because of color variation caused by variations in specific gravity, pellet size and flow characteristics of the color concentrate pellets through the metering equipment, the metering equipment needed has become more complicated and consequently more expensive. Even with this more expensive metering equipment, color uniformity during extended processing runs is still not entirely satisfactory using metered color concentrate.
It is usually not desirable and sometimes not possible to mix different types of thermoplastic resins. It has been necessary, therefore, for the resin processor to have a large color concentrate inventory -- in essentially the same color for each of the various resins that he uses. For example, if the resin processor processes polyethylene, polystyrene and polypropylene and wants to make the same color red parts from each of these resins, it will probably be necessary for him to stock three different red color concentrates, one for each resin.
"Dry color" is the name conventionally given to color pigments treated or coated with dispersing aids, such as stearic acid. It is the conventional alternative to color concentrates. Dry color, also, can be added to clear resin by preblending with the color pellets or by metering into the clear pellet feedstream. Handling dry color is a dusty and dirty operation and the degree of dispersion of the color pigment in the finished processed article is not always satisfactory and it is difficult to achieve color uniformity in the parts produced.
As an alternative to color concentrates or dry color, resin processors sometimes use so-called precolored resin, supplied by the manufacturers of the resin. Precolored resin contains the desired amount of pigment already dispersed into the resin pellets so that articles made from it have the desired shade and color. However, precolored resin is sold at a substantial premium above clear resin prices and usually is available only for large quantity users.
With any of the above methods of coloring plastics changing the equipment from production of a part in one color to the same part in another color is time consuming, normally taking about an hour, and involves considerable waste plastic. The time consumed is important, since plastic processing machines are normally costed at $50 to $100 per hour of operating time.
Thus, coloring methods available to thermoplastic resin processors in the past have been inefficient, expensive or not completely satisfactory from the standpoint of achieving color uniformity.
Prior to those disclosed in the cross-referenced applications, liquid colorants were tried without much success. Some of these early liquid colorants "plated-out" on processing equipment parts such as the extruder screw, causing a build-up to the extent that the screw seized; some such colorants failed to give finished articles of uniform color, as evidenced by streaking or specking of the product; some proved incompatible with some resins, particularly polyolefins; some caused too great a loss of physical properties in the finished products; some of these liquid colorants settled-out, separated and stratified while being stored prior to use; and finally some products did not possess the proper rheological characteristics to permit precise metering.
The liquid colorant dispersions for plastics which are disclosed in the cross-referenced applications have overcome the disadvantages of the early liquid colorants. For example, they can be introduced by conventional metering devices, such as pumps, into the feedscrew area of an injection molder or extrusion machine. Such dispersions have consistently produced plastic products of uniform color, substantially free of specking and streaking. Also the liquid nature of these dispersions allows a plastic processor to eliminate a large inventory of various colored plastic resin pellets and stock only clear resin and relatively small amounts of liquid colorants.
Another advantage of these liquid colorant dispersions is that when the molder wishes to change the color of the product, the procedure is relatively simple. The simplest method involves disconnecting the feed hose to the pump, connecting a clean hose to the pump and immersing the other end of the clean hose in the new color. The new color will displace and push out the old color. The resin products will show some brief initial blending of the colors, but usually only four or five shots (in injection molding) or a few feet of plastic (in extrusion situations) need be scrapped before the new color appears fully.
The change from one color to another can be made in about 6-8 minutes, 10 minutes as a normal maximum, when using liquid colorants.
A liquid colorant dispersion, to be commercially acceptable, must have the following properties:
1. Stability -- must not substantially settle or separate into layers after standing at the extremes of commonly encountered ambient storage conditions, such as -40.degree. to 140.degree.F, for at least 2 months.
2. Wetting Properties -- at the recommended levels of use, must wet plastic pellets such that they do not clump or stick together.
3. Compatibility -- must be compatible with common thermoplastic resins (styrenes, styrene-acrylonitriles, styrene-butadiene, acrylonitrile-butadiene-styrenes, olefins, vinyls, urethanes, etc.) such that plastic parts containing the dispersions can be post-processed in a conventional manner, such as by painting, printing, heat sealing, coating or glueing. If a liquid colorant dispersion were incompatible with the resin in which it was used, for example, the dispersion components might migrate to the surface of the plastic article and interfere with post-processing steps. Also, the liquid colorant dispersions must not substantially adversely affect physical properties of the plastic resins which they color.
4. Rheological Properties -- must be such that dispersions can be conveniently introduced into plastic processing equipment by means of common pumping equipment. This requires that the absolute viscosity be in the range from about 300 to 35,000 centipoise, preferably 1,000-20,000; that the plastic viscosity be in the range from about 800 to 15,000 centipoise, preferably 1,000-10,000 centipoise, and that the yield be in the range from about 0.0 to 25 units, preferably 0.0-12 units. These values are obtained at 25.degree.C by using a Brookfield Viscometer, model RVT, and using spindle No. 7 obtaining readings at 50RPM and 20RPM. A straight line is drawn through these two points and the x-axis intercept is taken as representing the yield value. The difference between the intercept and the 50RPM value is multiplied by the appropriate spindle factor and is taken as the plastic viscosity. The 50RPM reading is multiplied by the appropriate spindle factor to obtain absolute viscosity. This method of calculation is discussed in "Industrial Rheology and Rheological Structures," by Henry Green, John Wiley and Sons, Inc., New York, 1949, p.18. The pumping behavior of the dispersions of this invention correlate well with yield and plastic viscosity. Yield is a conventional measure of tendency of the material to flow smoothly without forming cavities within itself.
The qualifying term "substantially" is used at several points above. For example, must not "substantially" settle. In each case what is meant is "not beyond conventionally accepted commercial limits." For example, must not settle beyond conventionally accepted commercial limits which would be known to one skilled in the art.
Liquid colorant dispersions within the general scope of the above parameters are disclosed in the cross referenced application. Generally those therein disclosed comprise calcium carbonate particles and fatty acid esters, particularly those of sorbitan. That application also disclose that polyoxyethylene alkyl, or alkylaryl, ethers can be used to replace up to 80% of the fatty acid ester.